Each of portable electronic devices such as a notebook, a PDA, a portable DVD, a camcorder, a digital camera and a mobile phone has a battery therein for a portable use. In order to charge and use the battery, it is required that the DC power having an adequate voltage be supplied through a connector of a proper shape. These portable electronic devices, however, have different voltages depending on not only the type of the device but also the manufacturer. The shape of the input unit for receiving the DC power also varies depending on each device. Accordingly, a user having various kinds of portable electronic devices must carry several power supply apparatuses with him or her, usually called adaptors, and has to purchase several adaptors for respective devices. This will cause the user to suffer greater economic loss.
In an attempt to overcome these problems, a universal power supply apparatus has been proposed. The universal power supply apparatus refers to a power supply apparatus applicable to various kinds of portable electronic devices using one main unit.
In such universal power supply apparatus, a detachable tip, a cable, etc., which are differently applied depending on the shape of an input terminal of a device, has to be first provided. A method for setting a voltage suitable for an electronic device to be used should be then provided.
In a conventional universal power supply apparatus, there is a method for setting a voltage of a device in a main unit using a dip switch, a rotary switch or a resistor having a special shape as disclosed in U.S. Pat. No. 5,347,211 and for providing a tip or cable detachable from the main unit and mated with the shape of the input terminal of an electronic device to be used. According to this method, however, a user has to perform two steps of finding the tip or cable that can be used in a portable electronic device and setting a proper voltage in the main unit. Accordingly, this method has a disadvantage that a user who is short of common sense on the electronic device has a difficulty in using the device.
In order to overcome this problem, U.S. Pat. Nos. 5,510,691, 6,604,177, 6,091,611 and so on disclose a method for providing a special unit for changing a voltage in the tip or cable. The special unit includes inevitably at least one electronic element such as a resistor, a condenser and a diode. This unit is usually called an attached circuit. In case where an electronic element is disposed in the cable, however, an expensive cable has to be used as the electronic device changes. Further, if the special unit is installed in the tip, between-the-main circuit and the attached circuit has to be connected by means of a long cable. Accordingly, the method for providing the special unit has disadvantages that its manufacture and test methods are difficult and stability of a voltage is also low.